In recent years, various liquid crystal-based displays have been used in personal computers, televisions, cellular phones, and so on. Since such liquid crystal displays themselves are not light-emitting devices, surface light sources, called backlights, are placed on the back side for illumination to enable the display. To meet the requirement that not only illumination itself but also uniform illumination over the screen should be provided, backlights have a surface light source structure called side light type or direct type. In particular, side light type backlights, which illuminate the screen from the side, are used in slim liquid crystal display applications for slimness or small size requiring notebook computers and so on.
In general, such side light type backlights use a light guide plate system that uniformly illuminates the whole of a liquid crystal display through a light guide plate for uniformly transmitting and diffusing light from a cold cathode fluorescent lamp as an illuminating light source placed on the edge of the light guide plate. In this illumination system, a reflector is provided around the cold cathode fluorescent lamp to use light more efficiently, and a reflecting plate is provided under the light guide plate to efficiently reflect, to the liquid crystal screen side, the light diffused by the light guide plate. This structure reduces the loss of light from the cold cathode fluorescent lamp and provides the function of brightening the liquid crystal screen.
On the other hand, large-screen applications such as liquid crystal televisions use a direct light system, because the edge light system does not promise the desired high screen brightness. In the direct light system, cold cathode fluorescent lamps are arranged in parallel under the liquid crystal screen and placed parallel to one another above a reflecting plate. A flat reflecting plate or a reflecting plate shaped like a semicircular arch along a part of the cold cathode fluorescent lamp is used.
Such a reflector or reflecting plate for use in surface light sources for liquid crystal screen (generically called “surface light source reflecting member”) is required to be a thin film and to have high reflection performance. Conventionally, a white pigment-containing film, a film containing fine voids inside, or a laminate of such a film and a metal plate, a plastic plate or the like has been used. In particular, the film containing fine voids inside is widely used, because it is highly effective in increasing brightness and has high uniformity. (Japanese Patent Application Laid-Open (JP-A) No. 6-322153 and JP-A No. 07-118433)
Concerning the film containing fine voids inside, a nucleating agent is added to form the fine voids. An acyclic olefin resin such as polypropylene or polymethylpentene has been used as the nucleating agent. Liquid crystal screen applications have been extended from conventional notebook computers to other various devices such as desktop personal computers, televisions, and cellular phone displays in recent years. As high definition images on the liquid crystal screen have been required, improvements have been made to enhance the liquid crystal screen brightness and to make the image clearer and highly visible and, therefore, high-brightness and high-power illuminating light sources (such as cold cathode fluorescent lamps) have been used.
However, when the above conventional film is used as a surface light source reflecting member such as a reflecting plate or a reflector, light from the illuminating light source is partially transmitted to the opposite side, because of its low light reflectivity, so that the brightness (luminosity) of the liquid crystal screen may be insufficient and that the lighting efficiency may be reduced by the loss of the light transmitted from the illuminating light source. Such a problem has been pointed out, and there has been a strong demand for an improvement in the reflectivity and opacity of the white film.
For this purpose, it is necessary to make a fine dispersion of the nucleating agent. However, there have been the problems that (1) in recent years, making a fine dispersion of the nucleating agent is reaching a limit; and that (2) if fine dispersion is achieved, voids cannot be stably formed or maintained in a film production process, because the nucleating agent has low stiffness or low deformation temperature.
In addition, if the film production conditions are changed (for example, the heat treatment temperature is lowered) to solve the problem (2), a new problem such as low dimensional stability may occur.